This invention relates generally to radiation-hard semiconductor devices and, more particularly, to radiation-hard field-effect transistors that remain radiation hard at cryogenic temperatures.
Semiconductor infrared photodetectors are often arranged in focal plane arrays and cryogenically cooled to very low temperatures to provide sensitive detection of infrared radiation for various types of space-based sensor systems. For example, a common infrared focal plane array design employs a hybrid configuration having a semiconductor infrared photodetector array and a semiconductor multiplexer array. The photodetector and multiplexer arrays are fabricated as separate electronic components to allow each semiconductor array to be individually optimized for a particular application. The multiplexer array operates in the cryogenically cooled environment of the photodetector array and utilizes metal-oxide semiconductor field-effect transistors (MOSFETs) to provide a read out of the detector signals.
For sensitive infrared detection in the harsh radiation environment of space, the MOS field-effect transistors must be radiation hard at cryogenic temperatures and operate with low noise, high dynamic range and low power dissipation. Radiation hardness is required in order to prevent a radiation-induced buildup of positive charges at the interface between the silicon substrate and a silicon dioxide (SiO.sub.2) dielectric layer formed on the surface of the substrate for passivation. This positive charge buildup forms an inversion layer which induces a leakage current that often destroys the isolation properties of the transistor.
Radiation hardness is ordinarily achieved through the use of either a radiation-hard field oxide or a p.sup.+ guard ring. Unfortunately, conventional radiation-hard field oxides lose much of their radiation hardness at cryogenic temperatures and p.sup.+ guard rings impose a large density penalty, degrade transistor performance, and require extra fabrication steps. Accordingly, there has been a need for a field-effect transistor that is radiation hard at cryogenic temperatures and is suitable for high density applications. The present invention is directed to this end.